Memory components and controllers that calibrate multiphase synchronous timing references

ABSTRACT

A first timing reference signal and a second timing reference signal are sent to a memory device. The second timing reference signal has approximately a quadrature phase relationship with respect to the first timing reference signal. A plurality of serial data patterns are received from the memory device. The transitions of the first timing reference and the second timing reference determining when transitions occur between the bits of the plurality of data patterns. Timing indicators associated with when received transitions occur between the bits of the plurality of data patterns are received from the memory device. The timing indicators are each measured using a single sampler. Based on the timing indicators, a first duty cycle adjustment for the first timing reference signal, a second duty cycle adjustment for the second timing reference signal, and a quadrature phase adjustment are determined and applied.

TECHNICAL FIELD

The present disclosure relates generally to information storage andretrieval and, more particularly, to calibrating the timing referencesignals that time the transfer of data and/or control signals betweenmemory system components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an embodiment of a memory system.

FIG. 2 is a timing diagram illustrating uncalibrated quadrature timingreferences used to transmit a calibration pattern.

FIG. 3 is a timing diagram illustrating when transitions between bits ofcalibration patterns are received.

FIG. 4 is a flowchart illustrating a method of calibrating.

FIG. 5 is a flowchart illustrating a method of determining timingadjustments.

FIG. 6 is a flowchart illustrating a method of operating a memorydevice.

FIGS. 7A and 7B are block diagrams illustrating embodiments of a memorysystem.

FIGS. 8A-8E are timing diagrams illustrating the calibration of timingreferences.

FIG. 9A is a timing diagram illustrating a transmission of sampledsignal values.

FIG. 9B is a timing diagram illustrating a transmission of calibrationregister values.

FIG. 9C is a timing diagram illustrating a loopback transmission ofsampled signal values.

FIG. 9D is a timing diagram illustrating transmission of sampled signalvalues and calibration register values.

FIG. 9E is a timing diagram illustrating a transmission of sampledsignal values.

FIG. 9F is a timing diagram illustrating a transmission of calibrationregister values.

FIG. 10 is a flowchart illustrating a method of calibrating.

FIGS. 11A and 11B are flowcharts illustrating methods of calibrating.

FIG. 12 is a flowchart illustrating a method of adjusting internaltiming references.

FIG. 13 is a flow diagram illustrating a method of calibrating.

FIG. 14 is a block diagram of a computer system.

DETAILED DESCRIPTION

Various embodiments described herein relate to a system includingintegrated circuit devices, for example, memory devices and/or at leasta memory controller device that controls such memory devices (andmethods of operation of these respective devices). In severalembodiments, as is described in more detail below, a multiphase timingreference (e.g., quadrature clocks) is incorporated to orchestrate thetransfer of data, and/or commands that specify memory operations,between memory devices and controller devices. The multiphase timingreferences, in various embodiments, are calibrated externally withrespect to the memory devices, and within the memory devices.

In a specific embodiment, at least two timing reference signals areprovided, in a system, to one or more memory devices. The timingreference signals are the same frequency, but one is delayed from theother by approximately ¼ of a cycle time of the timing referencesignals. Thus, the two timing reference signals have a quadrature phaserelationship or are “in quadrature.” It should be understood that theconditions necessary to be approximately “in quadrature” is applicationdependent and does not necessarily mean exactly ¼ of a cycle. Instead,depending on the tightness of timing budgets, and other factors, a givenapplication (or location in a system) allows for a certain range around¼ of a cycle, as well as a certain deviation in the duty cycles of thequadrature clocks and can still be considered to be “in quadrature” orhave a quadrature phase relationship.

In an embodiment, the timing reference signals are distributed tomultiple memory devices in a “fly-by” topology. In a “star” or “T”topology, the signals are routed to arrive at some or all of the memorydevices at substantially the same time. In a fly-by topology, signalsare routed such that they arrive at a first device, then a next device,(i.e., at least two memory devices) then the next, etc., in sequence orserial-like fashion. Accordingly, the flight times of these timingreference signals from the timing reference source to each of the memorydevices are skewed, and thus different. In addition, because each memorydevice receives the signals at a different location, the skew, dutycycle distortion, and phase distortion between the two timing referencesignals may be different for each memory device.

In an embodiment, distributing two lower frequency quadrature timingreference (a.k.a., one of clock or strobe) signals allows these lowerfrequency timing reference signals to arrive at each of the destinationmemory devices with more amplitude than a single timing reference signalbeing sent at twice the frequency. Because there are two edges for eachof the two quadrature timing references per cycle, and those edges arenot aligned between one timing reference signal relative to anothertiming reference signal, the quadrature timing references define fourinstants (or periods) per cycle which may be used to synchronize signalsinto, or out of, a memory device. A signal may be clocked in (or out) ofa device by each edge of both of the timing references. Thus,distributing two timing references in quadrature enables signals to beclocked in/out of devices at four times the frequency of the individualtiming reference signal, while adequate signal strength of the timingreference signals is maintained upon arriving at the devices. However,skew, duty cycle distortion, and phase distortion (a.k.a. skew) betweenthe two timing reference signals as they are received at a memorydevice, or distributed within a memory device, cause these four periodsto be unequal. Calibrating these timing references so that they haveapproximately 50% duty cycles, and a phase delay between them ofapproximately one-quarter of a cycle ensures that the four periods areall approximately the same length of time.

FIG. 1 is a block diagram illustrating an embodiment of a memory system.In FIG. 1, memory system 100 comprises memory controller 110 and memory120. Memory controller 110 includes driver 111, driver 112, clock adjust113, calibration control 119, and receive bitslice 118. Memorycontroller 110 also includes timing reference ports CKI and CKQ that aredriven by driver 111 and driver 112, respectively. Receive bitslice 118includes samplers 117, one of which is sampler 116, and receive clockadjust 115. Samplers 117 are for receiving signals from memory 120 via asignal port, DQ.

Memory controller 110 and memory 120 are integrated circuit typedevices, such as ones commonly referred to as a “chips”. A memorycontroller, such as memory controller 110, manages the flow of datagoing to and from memory devices, such as memory 120. For example, amemory controller may be a northbridge chip, an application specificintegrated circuit (ASIC) device, a load-reduction memory buffer, agraphics processor unit (GPU), a system-on-chip (SoC) or an integratedcircuit device that includes many circuit blocks such as ones selectedfrom graphics cores, processor cores, and MPEG encoder/decoders, etc.

Although a single memory 120 is shown, there may be multiple memorydevices or chips disposed on a memory module and coupled to the memorycontroller via a connector interface. Memory 120 can include a dynamicrandom access memory (DRAM) core or other type of memory cores, forexample, static random access memory (SRAM) cores, or non-volatilememory cores such as flash. Memory controller 110 and memory 120 may beinterconnected with each other in a variety of system topologiesincluding on a PC board (e.g., where the memory is on a module and thecontroller is socketed to the PC board, or in “die-down” arrangementwhere one or both of the chips are soldered to the PC board), stackedone on top of another and encapsulated in a single package or eachhaving separate package (package-on-package), both disposed on a sharedsubstrate, on an interposer, or even in a direct-attach arrangement. Inaddition, although the embodiments presented herein describe memorycontroller and one or more memory devices, the instant apparatus andmethods may also apply to chip interfaces that effectuate signalingbetween separate integrated circuit devices.

In an embodiment, the signals output by timing reference ports CKI andCKQ are periodic at a stable frequency and have an approximatequadrature phase relationship to each other. Because CKI and CKQ areperiodic, CKI and CKQ may be referred to as clock signals (and thusdrivers 111 and 112 may be referred to as clock drivers; receivers 121and 122 may be referred to as clock receivers). The sent (and received)signal values on CKI and CKQ per approximately ¼ of each CKI cycle isgiven in Table 1. In another embodiment, the signals output by timingreference ports CKI and CKQ may be one of respective intermittent clocksignals or strobe signals that maintain a quadrature relationship toeach other. In this embodiment, because CKI and CKQ are strobes, drivers111 and 112 may be referred to as strobe drivers and receivers 121 and122 may be referred to as strobe receivers.

TABLE 1 Part of cycle CKI CKQ 1^(st) quarter cycle 0 0 2^(nd) quartercycle 1 0 3^(rd) quarter cycle 1 1 4^(th) quarter cycle 0 1

Note that each of the quarter cycles given in Table 1 involve a uniquecombination of CKI and CKQ. Thus, the states of CKI and CKQ, or thetransitions between these states, can be used as timing references tocontrol the transmission or reception of other signals, such as DQ, at arate that is 4 times the cycle time of CKI and CKQ. Note further thatthe above example is specific to a quadrature embodiment; otherembodiments are readily derived. For example, a sextile embodiment wouldcomprise three timing signals offset in phase from each other by ⅙^(th)of a clock cycle, an octal embodiment would compress four timing signalsoffset by ⅛^(th) of a clock cycle, etc. For purposes of explanation, thesimpler quadrature embodiment will be described herein.

Under the control of calibration control 119, receive clock timingadjust 115 adjusts at least a delay of an input clock signal to producea timing reference signal (RCK) supplied to at least sampler 116. Theadjustments to RCK at least allow the timing of the edge that triggerssampler 116 to be swept through a range of timings. The range of timingRCK may be swept at least include enough of a range that sampler 116 cansample a signal that was output by memory 120 in response to each of thefour edges output on timing reference ports CKI and CKQ.

Calibration control 119 also controls a duty cycle adjust input(DC[0:M]) and a quadrature phase adjust input (DL[0:N]) of clock adjust113. Accordingly, calibration control 119 may control the duty cycle ofCKI or CKQ, and the quadrature phase delay between them. Calibrationcontrol 119 may control (or adjust) the duty cycle output by CKI or CKQ,and the quadrature phase delay between them based on the sampled valuesreceived from sampler 116.

Memory 120 includes receiver 121, receiver 122, pattern generator 124,transmit bit slice 128, serializer 125, and transmitter 123. Timingreference ports CKI and CKQ of memory controller 110 are operativelycoupled to memory 120 ports CKI and CKQ, respectively. Signal port DQ ofmemory controller 110 is operatively coupled to signal port DQ of memory120, respectively. Receiver 121 and receiver 122 of memory 120 receivethe CKI and CKQ signals, respectively, from memory controller 110.Receiver 121 and receiver 122 of memory 120 generate internal clocks orstrobes derived from the CKI and CKQ signals, respectively, receivedfrom memory controller 110. Under the control of commands operativelyreceived from calibration control 119 (e.g., calibration commands can besent from memory controller 110 to memory 120 via a command channelinterface, not shown), transmit bit slice 128 is placed into acalibration mode. In this mode, serializer 125 responds to patterngenerator 124, and ignores the normal read data path (e.g., from thememory core). In this mode, pattern generator 124 supplies a pluralityof calibration data patterns, one at a time, to serializer 125 whichthen outputs the serial calibration data stream to transmitter 123 whichsends it, via the ports of memory controller 110 and memory 120, tosamplers 117, and sampler 116, in particular. In an embodiment, patterngenerator 124 may supply a plurality of pre-defined calibration datapatterns, one at a time, to serializer 125. The selection of thepre-defined calibration data patterns being controlled by calibrationcontroller 119, or a state machine in memory 120 (not shown in FIG. 1).In another embodiment, pattern generator 124 may receive programmablecalibration data patterns from memory controller 110 which are storedand supplied to serializer 125.

In an embodiment, memory controller 110 and memory 120, using at leastthe elements described previously, may form a closed-loop system forcalibrating the duty cycles of CKI and CKQ, and the quadrature phasebetween them. The calibration control 119 controls pattern generator 124to output a first calibration bit pattern. Calibration control 119 alsocontrols clock adjust 113 to control CKI and CKQ to start with defaultduty cycles and quadrature phase adjustments. CKI and CKQ is received byreceivers 121 and 122, respectively, causing serializer 125 to outputthe first calibration bit pattern as a serial bitstream. The serialbitstream is sent via transmitter 123 and DQ and received at the inputof sampler 116. The serial bitstream output by serializer 125 is may berepeated continuously until pattern generator 124 is commanded bycalibration control 119 to output a different calibration bit pattern(e.g., a different calibration pattern, or normal read data transfers).

While serializer 125 is outputting the repeating calibration bitpattern, calibration control 119 controls receive clock timing adjust115 to sweep RCK over a range of timings that allow calibration control119 to determine the timing of a transition between bits in thecalibration pattern sent by memory 120, based on the samples taken bysampler 116. In other words, for a given timing control value (DL[0:N])sent to RCLK timing adjust 115, sampler 116 will sample the calibrationbit pattern at a given point in time. Sampling at this point in timewill result in sampler 116 resolving to a logic value (i.e., a “1” or a“0”) according to the point in the calibration bit pattern that is atthe input of sampler 116. As RCLK is swept through a range of timings,the logic value resolving at the output of sampler 116 (and thus beingsent to calibration controller 119) will change (i.e., from a “1” to a“0” or visa vice versa). The timing, or timing control value DL[0:N], ator near where this change occurs may be associated with a transition onat least one of CKI or CKQ.

Calibration controller 119 may use the various predetermined calibrationbit patterns output under its control, and sweep ranges, to determinetiming indicators (e.g., DL[0:N] values) associated with each of thefour quadrature clock edges. It should be noted that calibrationcontroller 119 is able to determine these timing indicators using thesame receive clock adjustment circuit 115 and the same sampler 116. Thishelps reduce errors due to processing or circuit differences that wouldbe present if different samplers were used to measure timing indicatorsassociated with different edges. The timing indicators may be used todetermine duty cycle adjustments (e.g., DC[0:M] values) and quadraturephase adjustments (e.g., DL[0:N]) that are sent to clock adjust 113.

It should be understood that signal port DQ of both memory controller110 and memory 120 may correspond to any input or output pins (a.k.a.,pads, or balls, etc.) of memory controller 110 or memory 120 that relyon timing reference signals communicated via timing reference ports CKIand CKQ for synchronization. For example, signal port DQ can correspondto bidirectional data pins (or pads) used to communicate read data frommemory 120 to memory controller 110. Furthermore, it should beunderstood that the electrical signaling used by the DQ port may beeither single-ended (where one signal is electrically transported withone wire) or differential (where one signal is electrically transportedwith two wires), utilizing whatever voltage levels are suitable for thechosen signaling type. It should also be understood that a typicalmemory interface has multiple DQ signal ports in parallel between memorycontroller 110 and memory 120; resulting distinctions between per-deviceand per-bit timing calibration are discussed in more detail below.

FIG. 2 is a timing diagram illustrating uncalibrated quadrature timingreferences used to transmit a calibration pattern. The signals andtiming illustrated in FIG. 2 may correspond to signals and timing ofmemory system 100 (shown in FIG. 1). In FIG. 2, CKI is shownperiodically cycling with a period of T_(CYC) (i.e., a frequency of1/T_(CYC)). CKQ is likewise shown cycling with a period of T_(CYC). CKIis shown with a distorted (i.e., non 50%) duty cycle. Thus, the timethat CKI is high (T_(CKI,H)) and the time that CKI is low (T_(CKI,L))are unequal (i.e., T_(CKI,H)≠T_(CKI,L)). Likewise, CKQ is shown with adistorted duty cycle. Thus, the time that CKQ is high (T_(CKQ,H)) andthe time that CKI is low (T_(CKQ,L)) are unequal (i.e.,T_(CKQ,H)≈T_(CKQ,L)).

At the start of the timing diagram in FIG. 2, CKI and CKQ are both shownat a logic low. This corresponds to the first quarter cycle given inTable 1. A first rising edge of CKI is shown at a time when CKQ remainslow. After CKI has risen, CKI is high and CKQ is low. This correspondsto the second quarter cycle shown in Table 1. The first rising edge ofCKI in FIG. 2 is shown corresponding to roughly the transition to dataD₀ on DQ. This transition corresponds to the arrival, as sent by memorycontroller 110, of the rising edge of CKI at serializer 125 causing theoutput of transmitter 123 to change.

At a time T_(Q1) after the first rising edge of CKI, a first rising edgeof CKQ is shown. After CKQ has risen, CKI is high and CKQ is high. Thiscorresponds to the third quarter cycle shown in Table 1. The firstrising edge of CKQ is shown corresponding to roughly the transition todata D₁ on DQ. This transition corresponds to the arrival, as sent bymemory controller 110, of the rising edge of CKQ at serializer 125causing the output of transmitter 123 to change.

At a time T_(Q2) after the first rising edge of CKQ, a falling edge ofCKI is shown. After CKI has fallen, CKI is low and CKQ is high. Thiscorresponds to the fourth quarter cycle shown in Table 1. The fallingedge of CKI is shown corresponding to roughly the transition to data D₂on DQ. At a time T_(Q3) after the falling edge of CKI, a falling edge ofCKQ is shown. The falling edge of CKQ is shown corresponding to roughlythe transition to data D₃ on DQ. After the falling edge of CKQ, itshould be noted that both CKI and CKQ are low. This corresponds to thefirst quarter cycle shown in Table 1.

A second rising edge of CKI is shown at approximately T_(Q4) after thefalling edge of CKQ. The second rising edge of CKI is showncorresponding to roughly the transition back to data D₀ on DQ.

When the quadrature clocks CKI and CKQ are calibrated, the rising andfalling edges of CKI and CKQ each occur approximately T_(QUAD) (whereT_(QUAD)=¼ T_(CYC)) apart from a rising or falling edge of the othersignal. When the rising and falling edges of CKI and CKQ occurapproximately T_(QUAD) apart, the bit time (T_(BIT)) for a particularsignal value (e.g., D₀, D₁, etc.) is also approximately T_(QUAD).However, as shown in FIG. 2, the unequal duty cycles of CKI and CKQ, andthe uncalibrated quadrature phase (i.e., T_(Q1)≠T_(QUAD)) result in thebit times for the calibration pattern being unequal and/or notapproximately equal to T_(QUAD). By measuring when the data of thecalibration pattern (i.e., D₀-D₃) arrives at sampler 116, indicators ofthe times T_(Q1)-T_(Q4) may be determined. These indicators may be usedby calibration control 119 to adjust the duty cycle of CKI, the dutycycle of CKQ, and the quadrature phase between them (i.e., T_(Q1)) untilT_(Q1)=T_(Q2)=T_(Q3)=T_(Q4)=T_(QUAD).

In an embodiment, the calibration patterns sent by serializer 125, andused to measure timing indicators associated with each edge ofquadrature clock CKI and CKQ, (from which T_(Q1)-T_(Q4) may be derived)involve having only one bit of the calibration pattern have having afirst logic value, and the rest have having a second logic value. Thisone bit is rotated through each of the bits of the calibration pattern.This set of calibration patterns is given in Table 2.

TABLE 2 Pattern # D₀ D₁ D₂ D₃ 1 1 0 0 0 2 0 1 0 0 3 0 0 1 0 4 0 0 0 1

FIG. 3 is a timing diagram illustrating when transitions between bits ofcalibration patterns are received. FIG. 3 also illustrates the patternsgiven in Table 2. As can be seen in FIG. 3, each calibration patterngiven in Table 2 is used to determine a timing indicator (FP₀-FP₃) eachassociated with an edge of either CKI or CKQ. As discussed previously,calibration control 119 may determine timing indicators FP₀-FP₃ bysweeping the timing of sampler 116's clock input to determine whichvalues input to receive clock timing adjust 115 are close to, but oneither side of, the transition between bits of the calibration pattern.Since each calibration pattern shown in Table 2 has only one bit in thepattern in a first state, (i.e., 1) and the rest in the second state(i.e., 0), each calibration pattern can be used to determine a timingindicator associated with one edge of CKI or CKQ.

FIG. 4 is a flowchart illustrating a method of calibrating. The stepsillustrated in FIG. 4 may be performed by one or more elements of memorysystem 100. In the first step of the flowchart, quadrature clocks aresent to a memory device (402). For example memory controller 110 maysend CKI and CKQ quadrature clocks to memory device 120. Calibrationcontroller 119 may set the inputs to clock adjust circuit 113 so thatthe duty cycle of CKI, the duty cycle of CKQ, and the quadrature phaseare not calibrated (i.e., at least one of T_(Q1), T_(Q2), T_(Q3), orT_(Q4) is not approximately equal to T_(QUAD)).

In the second step of the flowchart, a memory device is instructed tosend a data pattern (404). For example, calibration controller 119 maysend a command to, or set a register value in, memory 120 that causespattern generator 124 and serializer 125 to output a serial calibrationpattern on DQ. This serial calibration pattern may be received by memorycontroller 110. In the next step, a receive clock control settingassociated with a transition between bits is determined (406). Forexample, calibration controller 119 may sweep the control settings ofreceive clock timing adjust 115 to determine which setting is associatedwith a transition between bits of the calibration pattern sent by memory120. A control setting of receive clock timing adjust circuitcorresponds to a particular timing of the receive clock. Thus, a controlsetting of receive clock timing adjust 115 may be used as a timingindicator. The transition associated with the timing indicatordetermined in block 406 may depend on the data pattern sent by memory120. For example, to determine receive clock control setting FP₀, datapattern #1 may be sent and with its leading-edge transition associatedwith the rising edge of CKI, as illustrated in FIG. 3. Similarly, todetermine receive clock control setting FP₁, data pattern #2 may be sentand with its leading-edge transition associated with the rising edge ofCKQ, as illustrated in FIG. 3.

Flow proceeds to block 404 if there is another data pattern needed todetermine at least one more timing indicator. Flow proceeds to block 410if all of the data patterns needed in order to determine enough timingindicators to perform the calibration have been sent (408). A CKI dutycycle adjustment is determined based on the determined receive clockcontrol settings (410). For example, the difference between receiveclock control setting FP0, (which is associated with the rising edge ofCKI), and receive clock control setting FP2 (which is associated withthe falling edge of CKI) may be used to determine a duty cycleadjustment to be applied by clock adjust 113.

In the next step of the flowchart, a CKQ duty cycle adjustment isdetermined based on the determined receive clock control settings (412).For example, the difference between receive clock control setting FP1,(which is associated with the rising edge of CKQ), and receive clockcontrol setting FP3 (which is associated with the falling edge of CKQ)may be used to determine a duty cycle adjustment to be applied by clockadjust 113. Finally, a quadrature phase adjustment is determined basedon the determined receive clock control settings (414). For example, thedifference between receive clock control setting FP0, (which isassociated with the rising edge of CKI), and receive clock controlsetting FP1 (which is associated with the rising edge of CKQ) may beused to determine a quadrature phase adjustment to be applied by clockadjust 113. Duty cycles and quadrature phase are checked to determine ifthey are within desired limits or ranges (416). If any of the dutycycles or the quadrature phase are not within desired limits, flowproceeds back to box 404 for further adjustment of the quadrature phaseand/or duty cycles. If all of the duty cycles and the quadrature phaseare within the desired limits, flow terminates in box 418.

FIG. 5 is a flowchart illustrating a method of determining timingadjustments. The steps illustrated in FIG. 5 may be performed by one ormore elements of memory system 100. In the first step, a first timingreference signal is sent to determine the timing of transitions betweenbits sent by a memory device (502). For example, memory controller 110may send CKI to memory 120. Memory 120 may use the CKI signal to clockserializer 125. Next, a second timing reference signal is sent todetermine the timing of transitions between bits sent by a memory device(504). For example, memory control 110 may send CKQ to memory 120.Memory 120 may use the CKQ signal to clock serializer 125.

In the next step of the flowchart, the timings associated with a firsttwo transitions between the bits sent by the memory device are measuredusing a single sampler (506). For example, memory controller 110 maymeasure the timing associated with the rising edge transition betweenbits of pattern #1 and pattern #3 of Table 2, as sent by memory 120 inresponse to transitions on CKI. These timings can be associated with therising and falling edges, respectively, of CKI. The timings associatedwith a second two transitions between the bits sent by the memory devicea measured using the single sampler (508). For example, memorycontroller 110 may measure the timing associated with the rising edgetransition between bits of pattern #2 and pattern #4 of Table 2, as sentby memory 120 in response to transitions on CKQ. These timings can beassociated with the rising and falling edges, respectively, of CKQ.

Finally, duty cycle adjustments or phase adjustments are determinedbased on the measured timings (510). For example, the first twomeasurements may be used to determine the current duty cycle of CKI.Once the current duty cycle is known, adjustments may be sent to clockadjust 113 which equalize the amount of time CKI is high (T_(CKI,H)) andlow (T_(CKI,L)). In another example, the second two measurements may beused to determine the current duty cycle of CKQ. Once the current dutycycle is known, adjustments may be sent to clock adjust 113 whichequalize the amount of time CKQ is high (T_(CKQ,H)) and low (T_(CKQ,L)).In another example, one of the first two measurements and one of thesecond two measurements may be used to determine the quadrature phasebetween CKI and CKQ. These two selected measurements should correspondto the same edge of CKI and CKQ. In other words, the edges selectedshould correspond to the rising edge of CKI and the rising edge of CKQ,or the falling edge of CKI and the falling edge of CKQ. Once the currentquadrature phase is known, adjustments may be sent to clock adjust 113which equalize the amount of time CKI and CKQ are in each of four phases(i.e., T_(Q1)=T_(Q2)=T_(Q3)=T_(Q4)=T_(QUAD)).

FIG. 6 is a flowchart illustrating a method of operating a memorydevice. The steps illustrated in FIG. 6 may be performed by one or moreelements of memory system 100. In the first step of this flowchart, afirst timing reference signal, sent by a memory controller, is received(602). For example, memory 120 may receive CKI from memory controller120. In the second step, a second timing reference signal, having aquadrature phase relationship to the first timing reference signal, isreceived from the memory controller (604). For example, memory 120 mayreceive CKQ from memory controller 110.

In the last step of the flowchart, a plurality of calibration bitpatterns are sent to the memory controller over a single linesynchronously with respect to the first timing reference and the secondtiming reference (606). For example, calibration controller 119 mayinstruct pattern generator 124 to output a plurality of bit patterns toserializer 125. Serializer 125 outputs a serial data stream of the bitpatterns synchronously with respect to transitions on CKI and CKQ. Theoutput of serializer 125 is carried to memory controller 110 (andsampler 116, in particular) via a single line, DQ.

FIG. 7A is a block diagram illustrating an embodiment of a memorysystem. In FIG. 7A, memory system 700 comprises memory controller 710Aand memory 720A. Memory controller 710A includes driver 711, driver 712,calibration control 719, drivers 713, and receiver 714. Memorycontroller 710A also includes timing reference ports CKI and CKQ thatare driven by driver 711 and driver 712, respectively.

Memory controller 710A and memory 720A are integrated circuit typedevices, such as one commonly referred to as a “chip”. A memorycontroller, such as memory controller 710A, manages the flow of datagoing to and from memory devices, such as memory 720A. For example, amemory controller may be a northbridge chip, an application specificintegrated circuit (ASIC) device, a graphics processor unit (GPU), asystem-on-chip (SoC), a memory buffer, or an integrated circuit devicethat includes many circuit blocks such as ones selected from graphicscores, processor cores, and MPEG encoder/decoders, etc. Memory 720A caninclude a dynamic random access memory (DRAM) core or other type ofmemory cores, for example, static random access memory (SRAM) cores, ornon-volatile memory cores such as flash. In addition although theembodiments presented herein describe memory controller and components,the instant apparatus and methods may also apply to chip interfaces thateffectuate signaling between separate integrated circuit devices.

In an embodiment, the signals output by timing reference ports CKI andCKQ are periodic at a stable frequency and have an approximatequadrature phase relationship to each other. Memory 720A includesreceiver 721, receiver 722, global clock adjust 723, internal clockdistribution 724, global calibration registers 725, multiplexer (MUX)726, driver 727, and receive bitslices 730A. Bitslice 731A is an exampleof one of the receive bitslices 730A. Bitslice 731A comprises receiver732, samplers 734, local clock adjust 733, local calibration registers735, and multiplexer 736. Timing reference ports CKI and CKQ of memorycontroller 710A are operatively coupled to memory 720A ports CKI andCKQ, respectively. Drivers 713 of memory controller 710A are operativelycoupled to bitslices 730A. Receiver 721 and receiver 722 of memory 720Areceive the CKI and CKQ signals, respectively, from memory controller710A. Receiver 721 and receiver 722 of memory 720A are operativelycoupled to global clock adjust 723. Global clock adjust 723 outputsadjusted CKI and CKQ signals which are distributed internally to memory720A, and to bitslices 730A in particular, by internal clockdistribution 724.

Under the control of commands received from calibration control 719,global calibration registers 725 control global clock adjust 723 to makeduty cycle adjustments and quadrature phase adjustments of CKI and CKQbefore they are distributed by internal clock distribution 724. Alsounder the control of commands received from calibration control 719,local calibration registers 735 control local clock adjust to make dutycycle adjustments and quadrature phase adjustments of CKI and CKQ localto bitslice 731A before they are used by samplers 734. Globalcalibration registers may also control the input selected by multiplexer736. The input selected by multiplexer 736 determines which of samplers734 is sent to multiplexer 726. Multiplexer 726 sequentially selectswhich bitslice 730A is sending a data bit from its samplers 734 tocalibration control 719 via driver 727 and receiver 714. Accordingly,values driven by drivers 713 are sampled by the samplers 734 ofbitslices 730A and may be sent back to calibration control 719.

In an embodiment, memory controller 710A and memory 720A, using at leastthe elements described previously, may form a closed-loop system forcalibrating the duty cycles of CKI and CKQ, and the quadrature phasebetween them internal to bitslices 730A. The calibration control 719controls drivers 713 to output a calibration bit pattern. Thecalibration bit is received at the input of samplers 734. The outputs ofsamplers 734 may be sent through MUX 736, MUX 726, and driver 727 tocalibration control 719. Accordingly, the received values of thecalibration bit pattern, with receive timing determined by CKI and CKQinternal to bitslices 731A which were adjusted by global clock adjust723 and local clock adjust 733, may be sent back to calibration control719. Calibration control 719 may used these received versions to makechanges to global calibration registers 725 and local calibrationregisters 735.

FIG. 7B is a block diagram illustrating an embodiment of a memorysystem. In FIG. 7B, memory system 701 comprises memory controller 710Band memory 720B. Memory controller 710B includes driver 711, driver 712,handshake control 718, and drivers 713. Memory controller 710B alsoincludes timing reference ports CKI and CKQ that are driven by driver711 and driver 712, respectively. Thus, it should be understood thatmemory system 701 is similar to memory system 700 of FIG. 7A. Adifference between memory system 700 and memory system 701 is that, formemory system 701, calibration control 729 is included in memory 720Bwhereas, for memory system 700, calibration control 719 is included inmemory controller 710A.

Memory controller 710B and memory 720B are integrated circuit typedevices, such as one commonly referred to as a “chip”. A memorycontroller, such as memory controller 710B, manages the flow of datagoing to and from memory devices, such as memory 720B. For example, amemory controller may be a northbridge chip, an application specificintegrated circuit (ASIC) device, a graphics processor unit (GPU), asystem-on-chip (SoC), a memory buffer, or an integrated circuit devicethat includes many circuit blocks such as ones selected from graphicscores, processor cores, and MPEG encoder/decoders, etc. Memory 720B caninclude a dynamic random access memory (DRAM) core or other type ofmemory cores, for example, static random access memory (SRAM) cores, ornon-volatile memory cores such as flash. In addition although theembodiments presented herein describe memory controller and components,the instant apparatus and methods may also apply to chip interfaces thateffectuate signaling between separate integrated circuit devices.

In an embodiment, the signals output by timing reference ports CKI andCKQ are periodic at a stable frequency and have an approximatequadrature phase relationship to each other. Memory 720B includesreceiver 721, receiver 722, global clock adjust 723, internal clockdistribution 724, global calibration registers 725, and receivebitslices 730B. Bitslice 731B is an example of one of the receivebitslices 730B. Bitslice 731B comprises receiver 732, samplers 734,local clock adjust 733, local calibration registers 735, and multiplexer736. Timing reference ports CKI and CKQ of memory controller 710B areoperatively coupled to memory 720B ports CKI and CKQ, respectively.Drivers 713 of memory controller 710B are operatively coupled tobitslices 730B. Receiver 721 and receiver 722 of memory 720B receive theCKI and CKQ signals, respectively, from memory controller 710B. Receiver721 and receiver 722 of memory 720B are operatively coupled to globalclock adjust 723. Global clock adjust 723 outputs adjusted CKI and CKQsignals which are distributed internally to memory 720B, and tobitslices 730B in particular, by internal clock distribution 724.

In response to handshaking or control signals from handshake control718, calibration control 729 makes duty cycle adjustments and quadraturephase adjustment of CKI and CKQ before they are distributed by internalclock distribution 724, and local to each of bitslices 730B. In oneembodiment, handshaking may comprise a mode register set (MRS) commandsent by memory controller 710B to memory 720B. The MRS command mayconfigure memory 720B to be in a clock calibration mode. In someembodiments, the clock calibration mode may be specified to be completeafter a certain time limit has passed. When this time limit has passed,another MRS command may configure memory 720B to exit the clockcalibration mode. In other embodiments, memory 720B may exit the clockcalibration mode once calibration is satisfactorily completed. Memory720B may also exit clock calibration mode after indicating on a signalto memory controller 710B that calibration is complete. Calibrationcontrol 729, global calibration registers 725 control global clockadjust 723 to make global duty cycle adjustments and quadrature phaseadjustments of CKI and CKQ before they are distributed by internal clockdistribution 724. Also under the control of commands received fromcalibration control 729, local calibration registers 735 control localclock adjust to make duty cycle adjustments and quadrature phaseadjustments of CKI and CKQ local to bitslice 731B before they are usedby samplers 734. Global calibration registers may also control the inputselected by multiplexer 736. The input selected by multiplexer 736determines which of samplers 734 are sent to calibration control 729.Accordingly, values driven by drivers 713 are sampled by the samplers734 of bitslices 730B and may be sent to calibration control 729.

In an embodiment, the elements of memory controller 710B, using at leastthe elements described previously, may form a closed-loop system forcalibrating the duty cycles of CKI and CKQ, and the quadrature phasebetween them internal to bitslices 730B. Memory 710B controls drivers713 to output a calibration bit pattern and calibration control 729 tobegin a calibration process. The calibration bit pattern is received atthe input of samplers 734. The outputs of samplers 734 may be sentthrough MUX 736 to calibration control 729. Accordingly, the receivedvalues of the calibration bit pattern, with receive timing determined byCKI and CKQ internal to bitslices 731B which were adjusted by globalclock adjust 723 and local clock adjust 733, may be sent back tocalibration control 729. Calibration control 729 may use these receivedversions to make changes to global calibration registers 725 and localcalibration registers 735.

FIGS. 8A-8E are timing diagrams illustrating the calibration of timingreferences. In FIG. 8A, CKI and CKQ are uncalibrated. In other words, atleast one of T_(Q1), T_(Q2), T_(Q3), or T_(Q4) are not approximatelyequal. In addition, for example, memory controller 710A is driving atoggling bit pattern to bitslice 731A at the full data rate. However,initially the rising edge of CKI is not aligned to the rising edge ofthe toggling bit pattern. Memory controller 710A performs writelevelization in order to align the rising edge of CKI with the risingedge of the bit pattern. Write levelization is performed by memorycontroller 710A by varying data timing driven by drivers 713 relative toCKI driven by driver 711. This is illustrated in FIG. 8B.

After write levelization, memory controller 710A, based on valuesreceived from samplers 734, adjusts the duty cycle of CKI by writingvalues to global calibration registers 725 and local calibrationregisters 735. The adjustment to CKI is shown in FIG. 8C where T_(CKI,H)and T_(CKI,L) have been adjusted to be approximately equal. Calibrationcontrol 719 may determine the adjustment to CKI by sweeping the timingof CKI and examining the results captured by the sampler 734 that isassociated with the falling edge of CKI. This adjustment may be appliedby calibration control 719 writing values to local calibration registers735 and/or global calibration registers 725. These adjustments may beperformed iteratively until the duty cycles and/or quadrature phase iswithin a desired range.

After memory controller 710A adjusts the duty cycle of CKI, memorycontroller 710A adjusts the quadrature phase between CKI and CKQ. Theadjustment to the quadrature phase between CKI and CKQ is shown in FIG.8D where T_(Q1) and T_(Q2) have been adjusted to be approximately equal.Calibration control 719 may determine the adjustment to the quadraturephase between CKI and CKQ by sweeping the timing of CKQ and examiningthe results captured by the sampler 734 that is associated with therising edge of CKQ. This adjustment may be applied by calibrationcontrol 719 writing values to local calibration registers 735 and/orglobal calibration registers 725.

After memory controller 710A adjusts the quadrature phase between CKIand CKQ, memory controller 710A adjusts the duty cycle of CKQ. Theadjustment to the duty cycle of CKQ is shown in FIG. 8E where T_(CKQ,H)and T_(CKQ,L) have been adjusted to be approximately equal and therebyT_(Q1), T_(Q2), T_(Q3), and T_(Q4) have been adjusted to beapproximately equal. Calibration control 719 may determine theadjustment to the quadrature phase between CKI and CKQ by sweeping thetiming of CKQ and examining the results captured by the sampler 734 thatis associated with the falling edge of CKQ. This adjustment may beapplied by calibration control 719 writing values to local calibrationregisters 735 and/or global calibration registers 725.

FIG. 9A is a timing diagram illustrating a transmission of sampledsignal values. In an embodiment, memory controller 710A sends a commandto memory 720A that instructs memory 720A to send one or more valuessampled by sampler 734 to memory controller 710A. Since bitslices 730Aare receiving the toggling calibration data pattern, memory 720A sendsthe sampled values to memory controller 710A using a signal line thatis, in normal operation, a one-way bit (OWB). This is illustrated inFIG. 9 by CMD #1 being sent by memory controller 710A on acommand/address bus. Then, after a predetermined length of time (orclock cycles), T_(CALR), memory 720A drives the values captured bysamplers 734 on the one-way bit via driver 727. In an embodiment, theone-way bit may be, or be associated with, an error detection andcorrection (EDC) pin, Data Bus Inversion (DBI), or Data Mask (DM).

FIG. 9B is a timing diagram illustrating a transmission of calibrationregister values. In an embodiment, memory controller 710A sends acommand to memory 720A that informs memory 720A that values to bewritten to a calibration register (e.g., global calibration registers725 and/or one or more local calibration registers 735). After apredetermined length of time (or clock cycles), T_(CALW), memorycontroller 710A drives the values to be written to the calibrationregister (QO[0:N]), to memory 720A. It should be understood that thevalues to be written to the calibration registers may be driven on thedata line (DQ) associated with that calibration register. In thismanner, calibration register values may be sent to an individualbitslice 731A without further addressing or control commands. Thebitslice 730A intended to receive a particular calibration registervalue receives it directly on its receiver 732 and thus no addressing orselection is necessary.

In an embodiment, memory 720A may be configured with other memories in amemory rank. In other words, memory 720A may share a select signal(e.g., a chip select signal) and/or other command and control signals(e.g., C/A signals) with one or more other memories (not shown in FIG.7A). Therefore, memory 720A and these other memories are accessedsimultaneously. It should be understood that because no additionaladdressing or selection is required, the individual bitslices 731A ofindividual memories in the same rank may receive individual calibrationregister values without affecting the calibration values of othermemories in the same rank.

It should be understood that memory controller 710A may receive phasedata bits associated with a first bitslice via a second bitslice. Thiseliminates the need to stop driving a calibration bit pattern in orderto read values sampled by samplers 734. Typically, memory 720A hasmultiple modes used to send phase data bits back to memory controller710A. Each of these modes may be associated with a particular phase ofCKI or CKQ. In other words, memory 720A may have a mode that sends thephase data bits associated with the rising edge of CKI, the falling edgeof CKI, the rising edge of CKQ, and/or the falling edge of CKQ. Thus,for a given mode, memory 720A may only send back samples associated withone edge of the quadrature clocks. As the phase of the selected clockedge is swept, the data bits sent back to memory controller 710A maystart at a first solid logic value (e.g., a logic high), go metastable,then become a steady logic value the opposite of the first logic value(e.g., a logic low). By using multiple modes, for multiple clock edges,the data being sent by memory 720A back to memory controller 710A may besent at a lower speed than quadrature clocks CKI and CKQ are toggling.

FIG. 9C is a timing diagram illustrating a loopback transmission ofsampled signal values. FIG. 9C illustrates the alignment of the risingedge of CKI. It should be understood that a similar diagram can be drawnto illustrate the other modes of operation that calibrate the fallingedge of CKI, the rising edge of CKQ, and the falling edge of CKQ. In anembodiment, the samples taken by a first one of bitslices 730A (e.g.,DQ_(IN)) may be sent back to memory controller 710A using a driverassociated with a second one of bitslices 730A (e.g., DQ_(OUT)). In FIG.9C a sample associated with the rising edge of CKI is taken of theDQ_(IN) input. This is illustrated by arrow 901. The value sampled bythe bitslice 730A associated with DQ_(IN) (in this illustration, a logichigh) is sent to the bitslice associated with DQ_(OUT). This isillustrated by arrow 906. Arrow 906 terminates at a point of theDQ_(OUT) waveform where DQ_(OUT) is a logic high. Another sampleassociated with the rising edge of CKI is taken of the DQ_(IN) input.This sample is illustrated by arrow 902. The value sampled by thebitslice 730A associated with DQ_(IN) after the phase adjustment of CKI(in this illustration, still a logic high) is sent to the bitsliceassociated with DQ_(OUT). This is illustrated by arrow 907. Arrow 907terminates at a point of the DQ_(OUT) waveform where DQ_(OUT) is a logichigh.

In order to sweep the clock edge being used to sample, at some point intime, memory controller 710A sends a command to memory 720A that informsmemory 720A that values to be written to a calibration register (e.g.,global calibration registers 725 and/or one or more local calibrationregisters 735). After a predetermined length of time (or clock cycles),T_(CALW), memory controller 710A drives the values to be written to thecalibration register (QO[0:N]), to memory 720A. This is illustrated inFIG. 9C by the data QO[0] and QO[1]. After the calibration registershave been written internal to memory 720A, a timing of CKI, CKQ, or bothhas been adjusted. A sample associated with the adjusted rising edge ofCKI is taken of the DQ_(IN) input. This is illustrated by arrow 910. Thevalue sampled by the bitslice 730A associated with DQ_(IN) (in thisillustration, a logic low) is sent to the bitslice associated withDQ_(OUT). This is illustrated by arrow 911. Arrow 911 terminates at apoint of the DQ_(OUT) waveform where DQ_(OUT) is a logic low.

FIG. 9C may be better understood with reference to FIG. 10. FIG. 10 is aflowchart illustrating a method of calibrating. The steps illustrated inFIG. 10 may be performed by one or more elements of memory system 700.In the first step of the flowchart, a memory is set to place pins to becalibrated in a calibration mode (1002). For example, memory 720A ormemory 720B may place a group of pins in a calibration mode. Memorycontroller 710A or memory 720B may cause this group of pins to be placedin a calibration mode by setting one or more values stored in globalcalibration registers 725 or local calibration registers 735. In anembodiment, this group of pins is a portion of a bus (e.g., one-half ofa DQ[0:N] bus). This portion may be ½ or less of the pins associatedwith the bus.

In the next step of the flowchart, the memory is set to output phasedata bits associated with the edge to be calibrated (1004). For example,memory 720A or memory 720B may set a group of pins that are not in thecalibration mode set in block 1002 to output phase data bits. Memorycontroller 710A or memory 720B may cause this group of pins to outputphase data bits associated with one of the rising or falling edges ofeither CKI or CKQ by setting one or more values stored in globalcalibration registers 725 or local calibration registers 735. In anembodiment, the group of pins set to output phase data bits are eachadjacent to the pins set in block 1002. In this manner, the phase databit sampled by the pins in the calibration mode may be sent to aneighboring pin. This reduces the distance a phase data bit must becommunicated from the pin where it was sampled to a pin that is drivingit back to memory controller 710A or 710B.

In the next step of the flowchart, a repeating calibration pattern istransmitted to the pins being calibrated and phase data bits arereceived from pins not being calibrated (1006). For example, memorycontroller 710A or memory controller 710B may transmit a repeatingcalibration pattern to the pins in calibration mode. In an example, thisrepeating calibration pattern may be selected from the patterns given inTable 2. In another example, this repeating calibration pattern may be aseries of alternating 1's and 0's toggling at the quadrature clock edgerate (as illustrated in FIG. 9C).

In the next step of the flowchart, it is determined if the desiredcalibration was obtained (1008). If the desired calibration wasobtained, flow proceeds to block 1012. If the desired calibration wasnot obtained, flow proceeds to block 1010. If the desired calibrationwas not obtained, in the next step of the flowchart, a clock phase orduty cycle is adjusted (1010). For example, calibration control 719 orcalibration control 729 may adjust a clock phase or duty cycle bysetting values stored in local calibration registers 725 and/or globalcalibration register 735. After a clock phase or duty cycle is adjusted,in the next step of the flowchart, flow proceeds to block 1006.

If the desired calibration was obtained, in the next step of theflowchart, it is determined if all of the clock edges have beencalibrated (1012). If all of the clock edges have been calibrated, flowproceeds to block 1016. If not all of the clock edges have beencalibrated, flow proceeds to block 1014. If not all of the clock edgeshad been calibrated, in the next step of the flowchart, another edge isselected to be calibrated (1014). For example, after the rising edge ofCKI is calibrated, the falling edge of CKI may be calibrated. Aftercalibrating the falling edge of CKI, the rising edge of CKQ may becalibrated. After calibrating the rising edge of CKQ, the falling edgeof CKQ may be calibrated. After another clock edge is selected to becalibrated, in the next step of the flowchart, flow proceeds to block1004.

If all of the clock edges have been calibrated, in the next step of theflowchart, it is determined if all of the pins have been calibrated(1016). If all of the pins have been calibrated, in the next step of theflowchart, flow proceeds to end in block 1020. If not all of the pinshave been calibrated, flow proceeds to block 1018. If not all of thepins have been calibrated, in the next step of the flowchart, anothergroup of pins is selected for calibration (1018). For example, memory720A or memory 720B may select a portion of the bus (e.g., the otherone-half of the DQ[0:N] bus) for calibration that was previously notselected for calibration.

It should be understood that the pins set to output phase data bits(i.e., in block 1004) may only be set during the calibration of one ofthe rising or falling edges of either CKI or CKQ. In other words, theoutputting of phase data on pins not being calibrated may only beperformed for a write levelization step (e.g., calibrating the risingedge of CKI), but an internal finite state machine (e.g., calibrationcontrol 729) may receive the phase data via an on-chip path for the restof the clock calibration (e.g., calibrating the falling edge of CKI andthe rising and falling edges of CKQ).

FIG. 9D is a timing diagram illustrating the transmissions of sampledsignal values and calibration register values. Similar to FIG. 9A, aread command 920 is sent to memory 720A by memory controller 710A. Atsome time later (t_(CALR)), the read data 930 (i.e., the phase data bitswhich are associated with the samples taken by samplers 734) is drivenby memory 720A and received by memory controller 710A. Similar to FIG.9B, a write calibration values command 921 is sent to memory 720A bymemory controller 710A. At some time later (t_(CALW)), the values to bewritten into one or more calibration registers 931 are sent, on thelines associated with those calibration registers, by memory controller710A. It should be understood that while some signal pins may beunidirectional during normal memory operation (i.e. they may beconfigured to only receive write data or only drive read data), in someembodiments, each pin may support both transmit and receive functions inorder to help calibrate clock settings as described above with referenceto FIG. 10.

FIG. 9E is a timing diagram illustrating a transmission of sampledsignal values. In particular, FIG. 9E illustrates details of read data930. In an embodiment, the result of each data pin's phase detector isoutput in a serial sequence on a single pin (e.g. the EDC pin) asillustrated by the labels DQ0 through DQ7, DBI, and EDC in FIG. 9E. Inan embodiment, the phase detector outputs ca n be interpreted asindicating whether a clock edge is too early or too late.

FIG. 9F is a timing diagram illustrating a transmission of calibrationregister values. In particular, FIG. 9F illustrated details ofcalibration write data 931. In an embodiment, QOFF is a field specifyinga specific offset. Thus, in FIG. 9F a five bit field (QOFF0-QOFF4) isillustrated. The offset specified by the QOFF field may be any setting.In an embodiment, the QOFF field may specify a quadrature offset betweenCKI and CKQ, the duty cycle of CKI, or the duty cycle of CKQ.

In an embodiment, each pin receives the QOFF field for that pin. Thisallows the device and pin receiving the QOFF field to be addressedindividually (i.e., other devices and other pins do not necessarilyreceive the same QOFF field value simply because they are connected to asame C/A bus.) It should also be understood that in FIG. 9F, theillustration of the QOFF field being sent or received, by memorycontroller 710A, 710B or memory 720A, 720B, respectively, on DQ[3:0] ismerely an example. QOFF fields for other data group pins (i.e., not C/Apins) can be sent or received. Examples of other data group pins thatcan send or receive QOFF fields include EDC, TRS, DBI (Data BusInversion), DM (Data Mask), etc. It should also be understood that thismethod of addressing registers may be used for other device or pinspecific fields beyond those used for quadrature clock calibration.

FIG. 11A is a flowchart illustrating a method of calibrating. The stepsillustrated in FIG. 11A may be performed by one or more elements ofmemory system 700. The system is initialized (1102). For example, memorycontroller 710A may write initial values to global calibration registers725 and local calibration registers 735. The initialization step mayalso include such activities as impedance calibration, voltage reference(Vref) calibration; receiver offset calibration, built-in self test(BIST), etc. Optionally, CLK and DCLK are aligned (1104). Optionally,training is performed on a command/address bus (1006). This trainingallows commands and addresses to be sent to memory 720A.

Read levelization is performed (1108). After read levelization, thevalues in global calibration register 725 and local calibration register735 may not allow for full speed operation of bitslices 730A, but may begood enough to allow one of CKI or CKQ to operate to drive data tomemory controller 710A at a reduced (e.g., ½) data rate. Writelevelization is performed (1110). After write levelization, the memorycontroller data transmit timing and the values in global calibrationregister 725 and local calibration register 735 may not allow for fullspeed operation of bitslices 730A, but may be good enough to allow oneof CKI or CKQ to operate to clock data into samplers 734 at a reduced(e.g., ½) data rate.

DCLK quadrature calibration is performed (1112). For example,calibration controller 719 may set global calibration registers 725 andlocal calibration registers 735 to adjust CKI and CKQ, internal tobitslices 730A, to have 50% duty cycles, and a quadrature phase that isapproximately ¼ of the cycle time of CKI and CKQ. Calibration controllermay set global calibration registers 725 and local calibration registers735 based on data received from samplers 734 and sent to memorycontroller 710A.

Read calibration is performed (1114). Write calibration is performed(1116). These calibrations allow memory controller 710A and memory 720Ato exchange data at full speed. Then, normal operation is entered(1118).

FIG. 11B is a flowchart illustrating a method of calibrating. The stepsillustrated in FIG. 11B may be performed by one or more elements ofmemory system 701. The system is initialized (1102). For example, memorycontroller 710B may write initial values to global calibration registers725 and local calibration registers 735. Optionally, CLK and DCLK arealigned (1104). Optionally, training is performed on a command/addressbus (1106). This training allows commands and addresses to be sent tomemory 720B.

DCLK quadrature calibration is performed (1112). For example,calibration controller 729 may set global calibration registers 725 andlocal calibration registers 735 to adjust CKI and CKQ, internal tobitslices 730B, to have 50% duty cycles, and a quadrature phase that isapproximately ¼ of the cycle time of CKI and CKQ. Calibration controllermay set global calibration registers 725 and local calibration registers735 based on data received from samplers 734.

Read levelization and calibration is performed (1024). Write calibrationand levelization is performed (1126). These calibrations allow memorycontroller 710B and memory 720B to exchange data at full speed. Then,normal operation is entered (1118).

It should be understood, with reference to FIGS. 11A and 11B, that byhaving calibration control 729 on memory 720A instead of memorycontroller 710B, a rough read levelization (i.e., boxes 1108 and 1110)of the values in global calibration register 725 and local calibrationregister 735 which does not allow for full speed operation of bitslices730B may not be necessary.

FIG. 12 is a flowchart illustrating a method of calibrating. The stepsillustrated in FIG. 12 may be performed by one or more elements ofmemory system 700. A first and second timing reference signals, with anapproximate quadrature phase relationship to each other, are received(1202). For example, memory 720 may receive uncalibrated CKI and CKQsignals from memory controller 710.

A plurality of signal values from a plurality of sampler circuits arereceived. The plurality of sampler circuits are triggered based on oneof the first, second, third, and fourth transitions defined by thequadrature relationship of the first and second timing referencecircuits (1204). For example, memory 720 may sample, using samplers 734,values in each bitslice 730 that are associated with an edge of CKI orCKQ. The values may be received at the inputs to MUX 726. In anembodiment, a value written by memory controller 710 to globalcalibration registers 725 or local calibration registers 735, sets thecontrol input of MUX 736. In another embodiment, a dedicated commandsent to memory 720 determines the control input of MUX 736. MUX 736determines which sampler 734 output in each bitslice 730 is selected tobe sent to memory controller 710. In another embodiment, all of thesampler 734 outputs are sent to memory controller 710.

The plurality of signal values are sent to a memory controller (1206).For example, memory 720 may sweep the value at the control input of MUX726 so that the values output by each bitslice are serially sent tomemory controller 710. In an embodiment, this is performed in responseto a command from memory controller 710.

A command that is based on the plurality of signal values is receivedfrom the memory controller. This command is to adjust an internalversion of the first and second timing reference signal (1208). Forexample, a command to set at least one value in global calibrationregisters 725 or local calibration registers 735 is received by memory720. The value set may affect an internal duty cycle or quadrature phaseof an internal version of CKI and/or CKQ.

The steps illustrated in FIG. 12 may also be performed by memory system701. However, it should be understood, that because calibration control729 is included in memory 720B, steps 1202, 1204, and 1208 may beperformed by memory system 701 without performing step 1206.

FIG. 13 is a flow diagram illustrating a method of calibrating. Theflows and steps illustrated in FIG. 13 may be performed by one or moreelements of memory system 700. Initial values are sent from memorycontroller 710A to global calibration registers 725 and/or localcalibration registers 735. Memory controller 710A sends quadratureclocks (i.e., CKI and CKQ) to memory 720A which are in turn, internal tomemory 720A, sent to receiver bitslices 730A. Memory controller 710Asends a calibration pattern (e.g., 1010101 . . . ) to bitslices 730A. Inresponse to the quadrature clock signals, receiver bitslices sample andsend sampled values of the calibration pattern to a driver 727. Thedriver 727 sends the sampled values to memory controller 710A.

Based on the sampled values, memory controller 710A send globalcalibration values to global calibration registers 725. After the globalcalibration values are received, receiver bitslices sample and sendsampled values of the calibration pattern to driver 727. The driver 727sends the sampled values to memory controller 710A. Based on the sampledvalues, memory controller 710A send local calibration values to localcalibration registers 735. From the foregoing, it should be understoodthat regardless of where the calibration control state machine islocated (i.e., memory controller 710A or memory 720B) having a globalclock adjust (e.g., global clock adjust 723) allows errors common to allbitslices 730A and 730B to be corrected with a smaller number ofadjustments than would be typical if all adjustments were performedinside of each individual bitslice 730A and 730B.

The methods, systems and devices described above may be implemented incomputer systems, or stored by computer systems. The methods describedabove may also be stored on a computer readable medium. Devices,circuits, and systems described herein may be implemented usingcomputer-aided design tools available in the art, and embodied bycomputer-readable files containing software descriptions of suchcircuits. This includes, but is not limited to memory systems 100, 700and 701, memory controllers 110, 710A, and 710B and memories 120, 720A,and 720B, and their components. These software descriptions may be:behavioral, register transfer, logic component, transistor and layoutgeometry-level descriptions. Moreover, the software descriptions may bestored on storage media or communicated by carrier waves.

Data formats in which such descriptions may be implemented include, butare not limited to: formats supporting behavioral languages like C,formats supporting register transfer level (RTL) languages like Verilogand VHDL, formats supporting geometry description languages (such asGDSII, GDSIII, GDSIV, CIF, and MEBES), and other suitable formats andlanguages. Moreover, data transfers of such files on machine-readablemedia may be done electronically over the diverse media on the Internetor, for example, via email. Note that physical files may be implementedon machine-readable media such as: 4 mm magnetic tape, 8 mm magnetictape, 3-½ inch floppy media, CDs, DVDs, and so on.

FIG. 14 illustrates a block diagram of a computer system. Computersystem 1400 includes communication interface 1420, processing system1430, storage system 1440, and user interface 1460. Processing system1430 is operatively coupled to storage system 1440. Storage system 1440stores software 1450 and data 1470. Storage system 1440 may include oneor more of memory systems 100, 700 and 701, memory controllers 110, 710Aand 710B, or memories 120, 720A and 720B. Processing system 1430 isoperatively coupled to communication interface 1420 and user interface1460. Computer system 1400 may comprise a programmed general-purposecomputer. Computer system 1400 may include a microprocessor. Computersystem 1400 may comprise programmable or special purpose circuitry.Computer system 1400 may be distributed among multiple devices,processors, storage, and/or interfaces that together comprise elements1420-1470.

Communication interface 1420 may comprise a network interface, modem,port, bus, link, transceiver, or other communication device.Communication interface 1420 may be distributed among multiplecommunication devices. Processing system 1430 may comprise amicroprocessor, microcontroller, logic circuit, or other processingdevice. Processing system 1430 may be distributed among multipleprocessing devices. User interface 1460 may comprise a keyboard, mouse,voice recognition interface, microphone and speakers, graphical display,touch screen, or other type of user interface device. User interface1460 may be distributed among multiple interface devices. Storage system1440 may comprise a disk, tape, integrated circuit, RAM, ROM, EEPROM,flash memory, network storage, server, or other memory function. Storagesystem 1440 may include computer readable medium. Storage system 1440may be distributed among multiple memory devices.

Processing system 1430 retrieves and executes software 1450 from storagesystem 1440. Processing system 1430 may retrieve and store data 1470.Processing system 1430 may also retrieve and store data viacommunication interface 1420. Processing system 1430 may create ormodify software 1450 or data 1470 to achieve a tangible result.Processing system 1430 may control communication interface 1420 or userinterface 1460 to achieve a tangible result. Processing system 1430 mayretrieve and execute remotely stored software via communicationinterface 1420.

Software 1450 and remotely stored software may comprise an operatingsystem, utilities, drivers, networking software, and other softwaretypically executed by a computer system. Software 1450 may comprise anapplication program, applet, firmware, or other form of machine-readableprocessing instructions typically executed by a computer system. Whenexecuted by processing system 1430, software 1450 or remotely storedsoftware may direct computer system 1400 to operate as described herein.

The above description and associated figures teach the best mode of theinvention. The following claims specify the scope of the invention. Notethat some aspects of the best mode may not fall within the scope of theinvention as specified by the claims. Those skilled in the art willappreciate that the features described above can be combined in variousways to form multiple variations of the invention. As a result, theinvention is not limited to the specific embodiments described above,but only by the following claims and their equivalents.

What is claimed is:
 1. A memory device, comprising: an interface; aregister to receive a first timing adjustment value for a first internaltiming reference signal, the first internal timing reference signalbased on a first external timing reference signal, the first timingadjustment value not allowing full speed operation of the interface; afirst circuit to receive the first external timing reference signal;and, a first sampler circuit, the first sampler circuit to resolve atleast two signal values, a first signal value of the at least two signalvalues to be associated with a first transition of the first externaltiming reference signal, a second signal value of the at least twosignal values to be associated with a second transition of the firstexternal timing reference signal.
 2. The memory device of claim 1,wherein the first timing adjustment value determines a duty cycle of thefirst internal timing reference signal.
 3. The memory device of claim 1,wherein the first internal timing reference is to be used by a pluralityof sampler circuits, the plurality of sampler circuits including thefirst sampler circuit.
 4. The memory device of claim 3, wherein a secondinternal timing reference based on the first internal timing referenceis to be used by the first sampler circuit, the second internal timingreference not to be used by the plurality of sampler circuits, a secondregister to determine a second timing adjustment of the second internaltiming reference signal.
 5. The memory device of claim 1, furthercomprising: a second circuit to receive a second external timingreference signal, the second external timing reference signal to haveapproximately a quadrature phase relationship with respect to the firstexternal timing reference signal, a third signal value of the at leasttwo signal values to be associated with a third transition of the secondexternal timing reference signal, a fourth signal value of the at leasttwo signal values to be associated with a fourth transition of thesecond external timing reference signal.
 6. The memory device of claim5, wherein the register receives a second timing adjustment value of asecond internal timing reference signal, the second internal timingreference signal based on the second external timing reference signal.7. The memory device of claim 6, wherein the first timing adjustmentvalue determines a first duty cycle of the first internal timingreference signal and the second timing adjustment value determines asecond duty cycle of the second internal timing reference signal.
 8. Thememory device of claim 7, wherein the register determines a third timingadjustment, the third timing adjustment determining a quadrature phasebetween the first internal timing reference signal and the secondinternal timing reference signal.
 9. The memory device of claim 5,wherein the first internal timing reference to be used by a plurality ofsampler circuits, the plurality of sampler circuits including the firstsampler circuit.
 10. A method, comprising: determining a first value fora first calibration register, the first value not allowing full speedoperation of the interface; setting the first value in the firstcalibration register; and, after setting the first value in the firstcalibration register, operating the interface at less than the fullspeed of the interface.
 11. The method of claim 10, further comprising:determining a second value for the first calibration register, thesecond value allowing full speed operation of the interface; and,setting the second value in the first calibration register.
 12. Themethod of claim 11, further comprising: after setting the second valuein the first calibration register, operating the interface at the fullspeed of the interface.
 13. The method of claim 12, further comprising:transmitting a first external timing reference signal; and, transmittinga second external timing reference signal, the second external timingreference signal to have approximately a quadrature phase relationshipwith respect to the first external timing reference signal, theapproximately quadrature phase relationship having first, second, third,and fourth transitions.
 14. The method of claim 12, further comprising:after setting the first value in the first calibration register andbefore setting the second value in the first calibration register,calibrating a first duty cycle of the first external timing referencesignal, a second duty cycle of the second external timing referencesignal.
 15. The method of claim 14, further comprising: calibrating aquadrature phase between the first external timing reference signal thesecond external timing reference signal.
 16. A method, comprising:receiving a first value for a first calibration register, the firstvalue not allowing full speed operation of an interface; and, aftersetting the first value in a first calibration register, operating theinterface at less than the full speed of the interface.
 17. The methodof claim 16, further comprising: receiving a second value for the firstcalibration register, the second value allowing full speed operation ofthe interface; and, setting the second value in the first calibrationregister.
 18. The method of claim 16, further comprising: after settingthe second value in the first calibration register, operating theinterface at the full speed of the interface.
 19. The method of claim17, further comprising: receiving a first external timing referencesignal; and, receiving a second external timing reference signal, thesecond external timing reference signal to have approximately aquadrature phase relationship with respect to the first external timingreference signal, the approximately quadrature phase relationship havingfirst, second, third, and fourth transitions.
 20. The method of claim16, further comprising: after setting the first value in the firstcalibration register and before setting the second value in the firstcalibration register, calibrating a first duty cycle of the firstexternal timing reference signal, a second duty cycle of the secondexternal timing reference signal, and a quadrature phase between thefirst external timing reference signal the second external timingreference signal.